The present invention relates to a reference electrode which may be used to measure a reference potential in various body fluids, such as blood, serum, cerebrospinal fluid or lymph. More particularly, the present invention relates to a reference electrode catheter which may be used in vivo for measuring such body fluids.
Reference electrodes for measuring various bodily fluids are well known. These electrodes generally include an outer housing member, an electrode positioned within the housing member, a liquid electrolyte disposed within the housing, and a porous liquid junction formed in one of the walls of the housing to provide an interface between the body fluid to be measured and the liquid electrolyte within the housing.
The porous liquid junctions have generally been formed of a porous ceramic material. One of the problems inherent in the use of porous ceramic junctions is the fact that certain body fluids tend to coagulate on the surface of the ceramic material and the pores in the ceramic material are very quickly blocked by various solid materials, such as proteins when the electrode is used for the measurement of blood or serum.
Another type of material has been proposed for the porous liquid junction. This material, takes the form of a hydrophilic polypropylene which is coated with a surface active agent. An example of a reference electrode which utilizes such a material for the liquid junction is disclosed in U.S. Pat. No. 4,177,126. Liquid junctions of polypropylene construction provide a satisfactory interface between the body fluids and the electrolyte within the reference electrode, for a limited period of time, however certain body fluids, such as blood, tend to coagulate on the surface of this material with a resultant change in the liquid junction potential after a period of time.
The sole function of a reference electrode is that of providing a very precise measurement of the reference potential of a fluid. Accordingly, any change or drift in the reference electrode results in corresponding inaccuracies in the total measurement taken of the fluid. As the pores in the liquid junction become clogged with organic substances, such as protein substances, the ion diffusion through the liquid junction is altered along with the associated liquid junction potential to thereby cause the electric signal developed by the reference electrode to change or drift. Even a slight drift in the signal, or slight instability in the reference electrode, results in inaccuracies in the total measurements taken of the body fluid.
When a reference electrode is utilized with a catheter for providing in vivo measurement of body fluids, it is imperative that the reference electrode exhibit the characteristic of having long term stability. It is particularly important that such long term stability be maintained in fluids, such as blood, which have a tendency to coagulate on the surface of any foreign body inserted into the fluid.